It is well known that oil field borehole evaluation may be performed by wireline conveyed instruments following the completion of the process of drilling a borehole Such techniques have been available to the oil field industry for decades Unfortunately, wireline investigation techniques are frequently disadvantageous due to their nature which requires that they be performed after drilling and after the pipe has been removed from the borehole Due to their inability to make their investigations in real time, they are unable to assist in the selection of casing, coring and testing points without significant delay. Additionally, while the wireline techniques are effective in determining formation parameters, they are unable to provide insight into the borehole drilling process itself.
In response to the shortcomings of wireline investigations, techniques which perform measurements while the borehole is being drilled are receiving greater acceptance by the oil field industry as standard, and indeed on occasion, indispensable services Many such techniques differ from the traditional wireline techniques in that the MWD techniques are able to measure drilling parameters which not only provide information on the drilling process itself but also on the properties of the geological formations being drilled. Due to the relatively recent increased use of many MWD techniques, the oil field industry is still in the process of learning from experience how to most effectively utilize the new information that is becoming available from MWD. Perhaps not surprisingly, accumulating experience is revealing some rather unexpected results that may significantly improve the knowledge and efficiency of the process of forming boreholes in the earth.
One recent example is described in U.S. Pat. No. 4,627,276 by Burgess and Lesso which is directed to a technique for remotely determining bit wear and for gaining insight into the efficiency of the drilling process from real time, in situ measurements of downhole weight on bit and downhole torque. Experience with this technique has shown that it is most effective in the drilling of boreholes in deltaic sedimentary geologies having shale beds occasionally interrupted by sandstone formations with milled-tooth bits Such a geology is found in the Gulf Coast region of the United States. Unfortunately, not all regions of the world have geologies as straight forward and as simple as the Gulf Coast Take for example the highly complex geology of California in which the pacific plate is thrusting itself under the continental plate to produce complex, highly fractured formations. In these difficult geologies, it has been discovered that the techniques of the aforementioned patent are difficult if not impossible to apply. Another geological example in which one would not expect the techniques of U.S. Pat. No. 4,627,276 to be effective is a volcanic geology. Thus, there is a need to discover and to develop methods of interpreting the measurements made while drilling complex geological formations that will bring some insight into the nature of the formations being drilled and the drilling process itself.
Such a clarifying technique has been discovered that reveals valuable and important information in the complex geologies of California and, by extension, probably in the simpler sedimentary formations as well. Contrary to expectation, it has been discovered that the drilling parameters of Rate of Penetration (ROP) and Downhole Torque (TOR) can be combined in a manner that not only may assist in identifying highly porous formations (highly fractured cherts in the California geology) but also may provide information on the undesirable drilling condition in which an undergauge or damaged bit is developed. The former is of major significance since in hard formations (such as chert) hydrocarbons tend to accumulate in fractures and the more highly fractured the formation, the greater the producibility of the stored hydrocarbons. The latter is also of major significance since the development of an undergauge bit means the diameter of the bit is slowly being reduced by abrasion of the formation on the bit to produce a slightly conical borehole which reduces in diameter with depth. As is well known, a conical borehole is a situation to be avoided, if at all possible, since it seriously magnifies the difficulty of performing subsequent operations in that section of borehole, such as continuing the drilling process with a full gauge bit or setting casing. When a conical borehole has been developed, expensive remedial actions to remove the tapering tendency of the borehole must be undertaken, such as reaming the borehole, before further activities can be resumed.